Ambrosiaster — on Gal 2:4 (EPISTLE TO THE GALATIANS 2.5.3-4)

“By "secretly" he means that they had entered by deception, passing themselves off as brothers when they were enemies. By "slipped in" he means that they came in a humble manner, feigning friendship.… To "spy out" is to enter in such a way as to invent one thing and discover another, whereby they may challenge our liberty.… "Liberty in Jesus Christ" means not being subject to the law. "That they might bring us into bondage" means … to subject us to the law of circumcision.”